Speakers Club: Chester Wallace

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Chester Wallace, of Windy Point Exploration, will give a talk at Speakers Club this Thursday entitled "The Role of fluid-expulsion structures in controlling stratabound-mineral deposits."

ABSTRACT

Stratabound mineral deposits form some of the richest metallic-mineral deposits on Earth. Annual production from the Lubin deposit in Poland exceeds 560,000 tonnes of refined copper from a sulfide deposit (10 m thick) in Mesozoic sandstone, which makes this mine one of the largest copper producer in the world. Recent advances in understanding the dynamics of evolving sedimentary basins allows predictions of locations of new stratabound-mineral deposits and predictions of the types of metals in stratabound-mineral deposits. These process-predictive methods integrate elements of sedimentary basin analysis with inorganic and organic chemistry, analysis of diagenetic minerals, determination of locales of hydrocarbon generation and later hydrocarbon migration, analysis of basin-wide overpressure, and geologic mapping of diagenetic phases.

Most sedimentary basins follow similar evolutionary sequences. As the basin subsides, pore fluid is first expelled from compacting sediment. Temperatures and pressures increase in the sediment during burial. Salinity increases in pore-fluid. During initial subsidence, compaction and “burial diagenesis” reduces permeability of the sediment. Metals leach from hydrocarbon source rocks in a predictable sequence as hydrocarbons are generated at burial depths of 500-3,000 m. Metal-bearing brine and hydrocarbons are expelled into conduits and the conduits that are overlain by seals (impermeable rock) provide migration pathways for metal-bearing oil-field brine and hydrocarbons. The oil-field brine contains inorganic and organic acids and metallic co-ordination compounds that permanently alter minerals in the conduit by dissolution, cementation, replacement, and metasomatism. These processes during later burial are collectively referred to as “alteration diagenesis.” At deep burial, alteration diagenesis creates secondary permeability, and fluid-migration pathways are formed. At about 3,000 m of burial, clay minerals in the sediment dehydrate, and these new fluids migrate from the deepest, highest pressure, and warmest parts of the basin toward shallower parts of the basin where lower pressures and temperatures prevail. These migrating fluids can also produce alteration diagenesis. Because metals leach from source rocks in a predictable order, the conveyer-belt of brine contains different metal endowments, which provides exploration predictability to resulting stratabound mineral deposits. Process-predictive analyses that result from reconstructed fluid-migration systems are central to locating hydrocarbon reservoirs and stratabound-mineral deposits.

We regard Fluid-Expulsion Structures (FES) as ancient equivalents to Seal-Bypass Systems (SBS) that are seen in 3-D seismic models in modern offshore sedimentary basins. Both FES and SBS represent pressure-control valves in subsiding sedimentary basins. FES and SBS occur where fluid pressure exceeds the strength of a seal. The integrity of the seal is forcefully broken and fluidized auto-breccia pierces upward through semi-consolidated sediment as breccia pipes. Cartwright and others (2007) estimated fluid-migration rates of 1-2 cm/s-1 through SBS, and they speculated that these pipes could remain open to migrating fluids for millions of years. The fluid that migrates through these pipes is composed of warm, acidic, reduced, metal-bearing oil-field brine, paraffin gases and gas condensate, and liquid petroleum.

The breccia pipe at Temple Mountain, Utah, on the eastern flank of the San Rafael Swell, is an exhumed FES that is a key to understanding and reconstructing fluid-migration systems in eastern Utah. The Temple Mountain structure is an elliptical pipe-like breccia mass, approximately 152 m wide and 610 m long (Keys and White, 1956) that cuts vertically 490 m through strata of the Moenkopi and Chinle Formations. Our interpretation, based on field study and analysis of drill-hole data, is that this breccia pipe formed when fluidized autobreccia intruded upward under high pressure from fluids in a conduit (White Rim Sandstone) through shale and siltstone that formed the regional seal (Moenkopi and Chinle Formations). In all ancient sedimentary basins FES regulated fluid pressure in conduits during basin subsidence.

Regional mapping of FES, conduits, seals, fluid-migration pathways, and structural compartments integrated with burial-history interpretations and basin evolution, supplemented by 3-D seismic models, will lead to a process-predictive basin model that can be digitized and animated. These process-predictive analytical methods can be applied to ancient sedimentary basins over the Earth.